This invention relates to biomedical systems for fluids separation embodying a rotor mounted on a pivot pin bearing, and is particularly concerned with medical disposable units involving rotation of a molded plastic part suspended on pivot pin bearings, especially for applications involving the rotation of a filtration support housing or for the rotation of a centrifugal separation housing. A particular area of usefulness of the present invention is in hemapheresis, relating to the separation of one or more constituents of blood, using disposable filtration or centrifugal separation devices. A specific area of application is in a plasmapheresis disposable separation device for the filtration of plasma from whole blood.
Plasmapheresis devices presently employed include membrane filtration devices as exemplified by the device disclosed and claimed in application Ser. No. 449,470, filed Dec. 13, 1982 for "Blood Fractionation System And Method" by H. Fischel. Such membrane filtration device has an interior spinner which is covered with a filter membrane and includes a conduit system for collecting the plasma passing through the membrane. The device receives an input fluid such as whole blood which passes into the space between the spinner and the outer wall of the housing. The plasma in the blood is filtered through the membrane into corrugations on the surface of the spinner, from which the plasma passes through holes into a central conduit and passes out at the bottom through an outlet conduit concentric with the central axis of the device.
The spinner is mounted in a closed cell or housing, and is driven remotely. This is accomplished by providing a magnetic coupling device at the top of the spinner. An external magnetic drive has one or more magnets which attract and lock onto the internal magnetic coupling device of the spinner. As the outer drive magnet is spun the inner magnetic coupling device is driven synchronously with the drive magnet to rotate the spinner, for example, at 3600 rpm. The magnetic coupling device may be either a piece of magnetic permeable material or another magnet oriented to attract the drive magnet as described in "Rotor Drive For Medical Disposables" by W. Williamson et al, Ser. No. 727,585, filed Apr. 26, 1985.
A pivot pin bearing is mounted at the top of the device to hold the upper end of the spinner during rotation, and a second pivot pin bearing is mounted at the bottom of the device on which the spinner rests during rotation. As the spinner is rotated by the external magnetic drive, blood passing into a gap or space between the rotating filter membrane of the spinner and an inner wall of the housing passes around the surface of the periphery of the membrane. As the blood passes through the gap plasma is filtered out into the interior of the spinner and packed blood cells which remain after plasma is removed are discharged from an outlet at the bottom or top of the housing. The plasma in the interior of the spinner passes through a bore in the lower pivot pin and is discharged from a central outlet port.
The spinner is rotatably supported by upper and lower pivot pin bearings and pivot pins which are conventionally made of stainless steel. An O-ring seal is loosely positioned between the spinner and the lower pivot pin bearing. In the conventionally employed stainless steel pin there are three important steps that must be performed carefully to insure the life of the seal. The first step involves machining a taper on the outside radius of the pin. This taper is required to prevent a steel edge from contacting and gouging an adjacent plastic wall when the pin rotates. The second involves the polishing of the end of the pin. This is accomplished by spinning the pin at a high rate of speed and applying a rubber abrasive for a certain length of time. Then the pin surface is brought to a mirror finish with a rouge polishing compound. The final part of this second step involves extensive cleaning of the pin to remove all polishing media and debris. The third step is the lubrication process. Both the pin and the O-ring are immersed in a diluted silicone oil solution. This silicone oil solution is specially tailored in both dilution and viscosity to provide sufficient lubrication without migrating to other undesirable areas of the blood separation filter. After all of the particulate matter has been cleaned from the pins, so that they are perfectly clean, the pins must be installed in devices and then sterilized by exposure to gamma radiation.
In order to avoid communication of disease the separation device is used only once and then discarded. Since the separation device is disposable it is desirable that it be manufactured as inexpensively as possible. However, since the upper and lower pivot pins for the rotor or spinner of these devices are fabricted from stainless steel, their initial raw material costs are relatively high. Further, the procedures for polishing, lapping the seal surface to minimize wear, and the cleaning of the pins greatly increases the cost of these pins. Moreover, the rejection rate for the processed stainless steel pins is surprisingly high, mainly due to the close tolerances which must be kept on the pivot pins in order for them to function properly together with the necessary tolerances required in molded plastic rotors.